1. Field of the Invention
The present invention relates to an ink jet recording head that forms images on a medium by enabling ink or other liquid to fly onto the medium.
2. Related Background Art
For the ink jet recording, ink or other liquid is caused to fly onto a medium for the formation of images. More specifically, it is arranged to discharge liquid from the discharge ports of an ink jet head as shown in FIG. 10. FIG. 10 is a front view which shows the recording device substrate of a side shooting type ink jet head.
In FIG. 10, reference numeral 1 designates a plurality of ink discharge ports arranged on the recording substrate for discharging ink; and 2, an orifice plate. An ink supply opening 3 is open substantially in the center of the recording element substrate on which the discharge ports are arranged to supply ink to the discharge ports.
This opening is usually made by means of sandblasting, anisotropic etching, laser processing, or the like.
FIG. 11 is a cross-sectional view taken along line 11--11 in FIG. 10. In FIG. 11, reference numeral 5 designates an electrothermal converting element; and 6, a nozzle portion that retains ink to be discharged by means of bubbling caused by heat generated by the electrothermal converting element 5.
The discharge ports are usually in a state of being exposed to the outside air. As a result, liquid in the nozzles is evaporated from the discharge ports, thus causing the viscosity of liquid in the nozzle portion to rise. In some cases, the phenomenon may take place that hinders the regular discharges of ink droplets.
FIG. 12 is a view which shows this phenomenon conceptually. The portion in the interior of a discharge port, which is indicated by slanted lines in FIG. 12, represents the state where the evaporating component in ink has been evaporated.
Here, on the portion indicated by the slanted lines, the viscosity has risen due to the fact that the density of the non-volatile component of a solvent or the like becomes more densified mainly because water is evaporated. Also, the ratio of colorant, such as dyes contained in ink, has increased in ink. (Hereinafter, ink in such state is referred to as viscosity-increased ink.)
When ink becomes viscosity-increased, the volume of ink discharge is reduced, the shooting accuracy is lowered, and the disabled discharges may take place, among some other drawbacks. Particularly, if the temperature and/or humidity of the environment of outside air is low, this phenomenon becomes more conspicuous.
Also, the increasing of density of dyes in the nozzle portion tends to higher density of prints at the start of printing, causing the unevenness thereof. Also, it has been found that the longer the interval between the last and current discharges, the more the evaporation is advanced, presenting these drawbacks more conspicuously.
Therefore, as means conventionally adopted, ink is discharged onto the region other than the recording area before printing or in printing, (which is hereinafter referred to as predischarge). The execution of the predischarges prevents these drawbacks from taking place in operation, because the ink whose water content has been evaporated to make it viscosity-increased is discharged from the nozzle portion beforehand.
Here, however, the frequent predischarges result in the increased amount of ink consumption, leading to the higher running costs.
Also, it is required to increase the capacity of the waste ink absorbing member for storing predischarged ink in an ink jet printer, which necessitates to make the size of the printer larger with the inevitable increase of costs.
Also, fundamentally, the volume of the nozzle portion of the ink jet head currently in use should become smaller as it is required to print images in higher resolution by use of smaller liquid droplets. As a result, the ratio of viscosity-increased ink contained in ink to be discharged tends to become larger. Here, it is known that once such smaller droplets are exposed to the outside air, the discharges are subjected more easily to the instability than the larger liquid droplets used conventionally.
In accordance with the conventional examples described above, the evaporation makes rapid progress in the ink jet head that discharges smaller droplets. This phenomenon may bring about the deviation in the shooting accuracy, the reduction of discharge volume, and the disabled discharges as well in some cases.
Also, with the attention given to a single discharge port, the longer the interval between the last and current discharges, the more the evaporation is advanced to aggravate the problems described above.